The present invention relates to an image processor and a color image processor for dithering multi-tone-level input image data to convert the data into data of fewer tone levels, used in a printer, a copier, a facsimile machine, an MFP (Multi-Function Peripheral) and the like.
Conventionally, a binary image output printer employing a line head, such as a line LED (light emission diode) head, a line thermal head and a line ink jet head, forms a binary image by printing dots coincident with the resolution of the head. Namely, if a line LED head is employed, dots having a size coincident with the distance between a plurality of recording elements (LED) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. If a thermal head is employed, dots having a size coincident with the distance between a plurality of recording elements (heating resistor) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. It is also well known to shift the head slightly in the direction of the main scanning to form an image on the same paper sheet repeatedly and to thereby realize a higher resolution than that corresponding to the distance between the recording elements.
In the image forming apparatus provided with the recording head of this type, a character/line image is reproduced as a binary image simply corresponding to the resolution of the head. A graphic/photograph image is reproduced as a binary image by a halftone processing such as an ordered dither method or an error diffusion method. In the halftone processing, it is difficult to both maintain a high resolution and reproduce a high tone level. In case of the ordered dither processing using the same threshold matrix repeatedly, In particular, resolution and tone properties are contradicting properties. The halftone processing is also used for color characters, shading colors and the like.
Further, as the image forming apparatuses provided with a recording head as stated above, there is proposed one for modulating the printing area of one pixel (or adjusting dot size) based on multi-level image data converted by the multi-level dither processing, thereby allowing expressing one pixel with several tone levels. An example of a recording head constituted by a plurality of recording elements used for such an apparatus as well as the state of dots is shown in FIG. 40. In FIG. 40, reference symbol 1 denotes a recording head, 2 denotes an ink discharge port and 3 denotes an output dot (printed dot).
For brevity, FIG. 40 illustrates an example of the output of dots of the image forming apparatus capable for expressing one pixel with three levels including white (output 0). In addition, by arranging four or three lines of these recording elements in parallel, it is possible to record a color image of C (cyan), M (magenta), Y (yellow) and K (black) or a CMY-color image.
The image forming apparatus capable of printing such multi-level image data conducts various image processings including a color conversion processing, a UCR (under-color elimination) processing and a gamma correction processing, to input RGB image data. Thereafter, the apparatus conducts a multi-level halftone processing such as a multi-level dither processing or a multi-level error diffusion processing employing screen angles for the respective colors so as to reproduce the number of tones intrinsic to a printer engine, to thereby obtain multi-level image data. The apparatus then outputs pixels having tone properties so as to enhance image reproducibility.
Generally, the ordered dither processing is relatively simple, has a high degree of freedom for configuration and has high processing speed, and the cost of the apparatus can be held down. However, it is said that the error diffusion processing is superior in image quality to the ordered dither processing. The ordered dither processing truncates quantization error in a comparison processing between input tone levels and thresholds, whereas the error diffusion processing diffuses quantization errors to peripheral pixels. Thus, they greatly differ in algorithm. As a result of the difference, compared with the ordered dither processing, the error diffusion processing can advantageously provide an output pattern having high frequency characteristics least conspicuous in light of human visibility, has a high edge holding effect and excellent image quality.
Oh the other hand, in a case of the halftone processing of a multi-level image output printer, it is known that the ordered dither processing and the error diffusion processing do not differ in image quality compared with the output of a binary image. This is because the truncated quantization error becomes far smaller than that of binary image data as the number of levels of the multi-level dither processing increases. In case of a high resolution printer, in particular, if the number of tones which one pixel can express is higher, the difference in image quality between the ordered dither processing and the error diffusion processing becomes less.
In addition, a method, such as a dither processing method employing fixed mask dither improved from stochastic dither or cluster dither, of realizing output characteristics comparable to that of the error diffusion processing at the same high speed as that of the ordered dither processing, is recently developed.
An ordinary binary output dither processing obtains binary output pixels by comparing input pixels with dither matrix thresholds at corresponding positions while basically, only taking into consideration a threshold array in a dither matrix on one plane. This state is shown in FIG. 41. FIG. 41 is a typical view showing a binary dither processing employing a well-known 4×4 Bayer dither matrix. To simplify description, input pixels of 4-bit tone level are compared with corresponding thresholds in a dither matrix. If the input tone level is equal to or higher than the corresponding threshold in the dither matrix, 1 (black) is output and if it is lower than the corresponding threshold, 0 (white) is output, thus obtaining a binary output image in combination of 1 and 0.
As shown in FIG. 41, the dither matrix has a configuration in which a unit dither threshold matrix of, for example, 4×4 (to be simply referred to as “unit matrix” hereinafter) is repeatedly used regularly and performs the above-described processings to all input pixels. Further, a normal output apparatus, such as a printer, often outputs a pixel similar to a circle rather than a square pixel due to the process limitations of the apparatus. The output state in this case is shown in FIG. 42. When all pixels are printed, the shapes of the printed pixels are designed to be ones completely covering ideal square pixels, i.e., circles with a diameter equal to or larger than √{square root over (2)} times as large as a resolution pitch like dot “1” shown in FIG. 42.
On the other hand, in the multi-level dither processing, it is necessary to consider not only a plane threshold array in the above-stated basic dither matrix but also depth (pixel level) direction. For example, in case of conducting a multi-level, e.g., N-level dither processing, (N−1) threshold planes are required. Dither thresholds on each of the threshold planes are compared with input tone levels, to thereby obtain an N-level output image. The state of this multi-level dither processing is shown in FIG. 43 and the state of output dots is shown in FIG. 44. FIGS. 43 and 44 show multi-level outputs including 0 (white).
Normally, in the dither processing, a high-quality image can be obtained if there is some sort of correlation among thresholds on a threshold plane and that among threshold planes. Accordingly, thresholds in (N−1) dither matrixes are often calculated automatically based on a reference threshold array indicating such correlation.
In the multi-level dither processing taking account of this correlation among planes, there are roughly two sequences of threshold arrays extending over the respective planes as shown in FIGS. 45A and 45B. To simplify description, FIGS. 45A and 45B show a multi-level dither processing for converting input 8-bit image data into an image of four levels per pixel (2 bits) using a 2×2 reference threshold array. FIG. 45C shows the reference threshold array. This reference threshold array indicates the order of the magnitudes of thresholds arranged on a threshold plane.
The sequence shown in FIG. 45A is to determine thresholds sequentially from the first plane. For example, all thresholds on the first plane are determined and then those on the second plane are determined. In conducting a dither processing using such threshold planes, if tone level “100” is input, for example, a pixel at a position corresponding to “1” of FIG. 45C is judged to have a tone level 2, a pixel at a position corresponding to “2” is judged to have a tone level 1, a pixel at a position corresponding to “3” is judged to have a tone level 1 and a pixel at a position corresponding to “4” is judged to have a tone level 1.
This sequence is used in a printer such as an ink jet printer, which is basically less influenced by the appearance state, i.e., presence/absence of neighboring pixels or tone levels thereof and which can stably form an image out of independent pixels. The resolution of an image output using this sequence is very high and almost comparable to that of a printer engine. Thus, this sequence is ideal for reproducing an image by means of area modulation. However, if an input image has a uniform tone level, pixels of the same or similar size are easily filled to form an output image. Due to this, the image is susceptible to the print position accuracy or the printing accuracy, such as dot size accuracy, of the apparatus.
In the sequence shown in FIG. 45B, thresholds at corresponding positions on planes are sequentially determined from the first to the third planes. For example, thresholds at a position corresponding to “1” shown in FIG. 45C, i.e., “20” on the first plane, “39” on the second plane and “59” on the third plane are determined, and then thresholds on the first to third planes at positions corresponding to “2” shown in FIG. 45C are determined. In conducting a dither processing using these threshold planes, if tone level “100” is input, for example, a pixel at a position corresponding to “1” of FIG. 45C is judged to have tone level 3, a pixel at a position corresponding to “2” is judged to have a tone level 2, a pixel at a position corresponding to “3” is judged to have a tone level 0 and a pixel at a position corresponding to “4” is judged to have a tone level 0.
This sequence is used in a printer, such as a laser printer and a thermal printer, which tends to be influenced by the appearance state of neighboring pixels and for which it is difficult and unstable to form an image out of independent pixels. With this sequence, a printed image has low resolution and low dot concentration. If a dither threshold array is formed as a dot concentrate type array, i.e., formed such that a plurality of dots are printed in block, an image called a mesh-dot image is formed. This mesh-dot image represents an image having points orderly arranged like a mesh while the block of dots constitute one point. Since the printer of this type is low in resolution, minor print position error in units of pixels is inconspicuous.
In either of the above two examples, all the thresholds arranged on the respective planes are automatically calculated when the reference threshold array and the threshold sequence among planes in depth direction are defined.
Next, consideration will be given to the relationship between the mechanical accuracy and printing accuracy such as print position, print size and the like of a plurality of recording elements constituting a recording head in a printer provided with the recording head as in the case of the ink jet printer. In case of the ink jet printer, for example, the volume and accuracy of the jetting direction of ink discharged from nozzles serving as recording elements, normally differs according to the nozzles. Although it is possible to enhance this printing accuracy to the extent that image quality is not adversely influenced all, production cost disadvantageously rises.
Further, if the print head is slightly shifted in main scanning direction and images are formed on the same paper sheet a plurality of times to thereby realize a higher resolution than that corresponding to the distance between the recording elements, the positions of the images to be formed may be possibly shifted from their respective target positions. To completely correct the shifts requires quite high accuracy in mechanical control, which again disadvantageously pushes up cost.
If the discharge volume and discharge direction of ink vary with nozzles, such an image as shown in FIG. 46 is output. Namely, FIG. 46 shows an image if all of the recording elements are driven based on the same tone level. As shown therein, a portion having a large dot (nozzle) and that to which neighboring dots are closer, are higher in concentration than the other portions and a black stripe occurs. Also, a portion having a small dot (nozzle) and that to which neighboring dots are more distant are lower in concentration than the other portions and a white strip occurs. As can be seen, if the discharge volume and discharge direction of ink vary with nozzles, concentration becomes uneven, resulting in the deterioration of image quality.
To avoid this, there is adopted a method, such as checked, thinning printing, of alternately printing lines in the same sub-scanning direction with a plurality of different nozzles without printing them with the same nozzle and thereby reducing the unevenness of concentration and the influence of stripes. According to this method, it is expected that the unevenness of concentration in the form of stripes can be reduced compared with a case where no countermeasures are taken. This method, however, has disadvantage of delaying printing speed proportionately with the complexity of the printing method.
Further, according to an image forming apparatus capable of expressing one pixel with a plurality of tones by modulating an area for printing one pixel while using multi-level image data, the unevenness of concentration due to printing error is inconspicuous in a relatively highlighted part (i.e., a region having small diameter dots and low concentration). However, if an image of a uniform tone level is reproduced on one plane with dots of medium or larger size to the extent that neighboring dots are almost in contact with one another, the unevenness of concentration in the form of stripes becomes particularly conspicuous. That is to say, a white stripe occurs to a portion from which neighboring dots are distant, whereas a deep stripe occurs to a portion in which dots overlap with one another. As for human visibility, in particular, the visible sensitivity of horizontal and perpendicular direction is very high. For that reason, it is quite likely that the unevenness of concentration in the form of stripes in horizontal and perpendicular directions are recognized as such even with slight positional error.
Furthermore, as for the formation of a color image, the tone reproduction characteristic comparable to the quality of a photograph including a highlight becomes increasingly important in recent color printers. The reproduction of tones capable of further reducing graininess is one of the most important technical challenges among others. Graininess indicates the degree of roughness in a printed image. A good graininess image indicates an image which tones change uniformly or smoothly and a bad graininess image indicates an image having conspicuous dots or roughness.
As a technique for satisfying the graininess, there is proposed a method of reducing the graininess of a highlight using thin ink colors, e.g., light cyan and light magenta beside standard four ink colors of C (cyan), M (magenta), Y (yellow) and K (black). With this method, however, the number of recording heads and driving mechanisms increase proportionately to the number of added ink colors. If a recording head has the same number of nozzles as that of pixels on a line per color, this disadvantageously leads to cost hike.
Moreover, color printing is faced by a problem of the unevenness of colors due to slight difference in the overlapping manner of the respective colors of C, M, Y and K. As for the four color printing of C, M, Y and K, various multi-level dither methods including dispersion dither methods represented by a halftone dot dither method and a Bayer dither method employing screen angles, a cluster dither method having intermediate characteristics between that of the dispersion dither method and the Bayer dither method and the like, have been developed.
For example, if the halftone dot method employing screen angles is applied to dithering, colors interfere with one another to thereby cause moiré such as roseate moiré. If a dispersion dither matrix such as a conventional Bayer matrix is employed, conspicuous texture appears at a specific tone part due to the low degree of freedom for the arrangement of dots. As can be seen, many problems still remain unsolved before obtaining optimum output characteristics over the entire colors or tones.
These problems with dither processing occur to both a binary output printer and a multi-level output printer employing a dither matrix. While the problems are particularly serious in the dither processing of the threshold sequence shown in FIG. 45B, they are not completely solved in the dither processing of the threshold sequence shown in FIG. 45A, either.
Additionally, while this is common to all these ordered dither processings including a cluster dither processing, periodicity tends to be easily seen over the entire tone ranges of input image data. The periodicity is particularly conspicuous in a printer with relatively low resolution.
Recently, a processing method of realizing output characteristics comparable to that of the error diffusion processing at as high speed as that of the ordered dither processing by employing fixed mask dither improved from the stochastic dither or cluster dither, is being developed. One preferred example of this method is described in Robert Unichney, “The Void-and-Cluster Method for Dither Array Generation”, SPIE/IS&T Symposium on Electronic Imaging Science and Technology, San Jose, Calif., February 1993. This processing method, however, assumes only theoretical output characteristics in an ideal system and it considers the dot overlapping model of a binary printer, i.e., the manner in which neighboring dots overlap with one another at best. According to this processing method, therefore, only the improvement of output characteristics can be expected.